Class II Human Leukocyte Antigens (HLA) regulate the overall immune response and also trigger the rejection of a transplanted organ. The specific aim of the proposed research is to downregulate the expression of HLA class II antigens (HLA) . The inhibition of expression of HLA class II genes will be achieved by transfecting cells with the recombinant molecules containing the cloned cDNA from the D region of the HLA complex in antisense orientation. These constructs will have the dominant selectable markers, hph or neo for selection or the mutant dihydrofolate reductase (DHFR*) gene for selection and amplification of the transgenomic fragments in mammalian cells. Expression vectors, episomal replicons and retroviral vectors will be used for the construction of the recombinant molecules. The molecules containing the insert will be transferred into the cells by electroporation in case of the expression vectors and episomal vectors and by infection when the retrovirus vectors are used as the means of gene transfer. Reduction in cell surface expression of class II antigens on the drug resistant cell lines will be evaluated by monoclonal antibody staining with flow cytometric determinations and by the ability to activate cell proliferation in the mixed lymphocyte culture. The long-term goal of the proposed project is to apply the technique of antisense cDNA to the downregulation of the expression of HLA class II antigens in bone marrow cells and to induce tolerance to transplantation of mismatched and haplocompatible bone marrow stem cells to the recipient. Non-human primates will be used for preclinical trials.